1. Field of the Invention
This invention relates generally to gas diffusion electrodes and, more particularly, this invention relates to gas diffusion electrodes adapted for use in electrochemical cells utilizing an aqueous alkaline electrolyte and consuming or generating a gas via the electrochemical process occurring within the gas diffusion electrode.
2. Description of Related Art
The use of gas diffusion electrodes in fuel cells and metal-air batteries is well known. Gas diffusion electrodes have also been used in the electrolysis, either oxidation or reduction, of gaseous reactants. It is also possible to generate gases in such electrodes. In general, gas diffusion electrodes take the form of solid porous (gas and liquid permeable) bodies formed at least in part of an electronically conductive, electrochemically active material, and may include a catalyst. Such electrodes generally define an electrolyte contacting surface and a gas contacting surface. Electrochemical oxidation and reduction occur at the points in the electrode where the gas to be oxidized or reduced contacts both the electrolyte and the active material of the electrode. In the case of gas generation, electrolyte contacts the active material, and gas is generated at this interface.
Electrochemical cells utilizing such electrodes generally comprise the gas diffusion electrode, a spaced counter electrode, a liquid electrolyte (which is generally aqueous) which contacts both the counter electrode and the gas diffusion electrode, and a gas which contacts the gas diffusion electrode either (1) for reduction or oxidation of the gas or (2) produced via electrolytic generation. Circuit connections are disposed between the counter and gas diffusion electrodes. Additionally, the counter electrode may also be a gas diffusion electrode. A well-known example of such a design is the H.sub.2 /O.sub.2 fuel cell.
Electrochemical batteries, for example, the metal-air type, commonly utilize either an aqueous alkaline or neutral (e.g., saline) electrolyte, while fuel cells may commonly utilize either acidic electrolytes or alkaline electrolytes. Other types of electrolytes are also used, depending upon the specific gas which is consumed or generated.
The use in electrochemical batteries of an oxygen-containing gas, such as air, which is reduced at the gas diffusion electrode is well known. However, the gas need not be oxygen-containing nor need it be reduced at the gas diffusion electrode. For example, hydrogen gas is oxidized in some fuel cells. The present invention is generally applicable to all such types of gas diffusion electrodes and cells.
The electronically conductive material in a gas diffusion electrode typically may be carbon. Additionally, a wide variety of catalysts, such as platinum or transition metal organometallic catalysts (such as porphyrins), are available.
In various applications, it is desirable that either or both the liquid electrolyte and the gaseous electrode reactant be flowed through the body of the cell over the electrode surfaces. Flowing electrolyte and/or flowed gaseous reactant are, of course, accompanied by a pressure drop across the cell, especially on the electrolyte side. This can be lead to excess pressures either on the gas-side or the electrolyte-side of the electrode. Furthermore, it may be desirable in certain circumstances to operate at an elevated gas pressure with respect to the electrolyte pressure. One example of such a situation would be one in which the performance is increased by pressurizing the gaseous reactant. In battery and fuel cell applications, it is desirable to obtain as high a cell voltage as possible at any given current density. One means of accomplishing this is to utilize a relatively high gas pressure or flow rate.
The use of a porous (e.g., typically 30%-60% porosity) gas diffusion electrode, however, poses difficult flow management problems. When gas pressure exceeds liquid electrolyte pressure by a sufficient amount, "blow-through" of gas through the electrode into the liquid electrolyte results. In conventional gas diffusion electrodes, this so-called "blow-through pressure" is usually much lower than is desirable for tolerance of substantial differential pressures between the gas and liquid sides of the cell.
For example, while it may be desirable to operate a cell at a gas vs. liquid differential pressure of up to 10 psi or more, typical air cathodes exhibit a gas blow-through pressure of less than about 0.25 psi. If the differential pressure exceeds the blow-through pressure, pumping of gas into the liquid electrolyte may result. (Typical blow-through pressures range from 0-1 psi, and are determined primarily by interfacial tension and pore size distribution.)
Conversely, if the liquid electrolyte pressure is higher than the gas pressure and the differential pressure exceeds the liquid bleed-through pressure, liquid may be pumped into the gas side of the cell, which may result in liquid in the gas manifold, with consequent pumping problems and a decrease in cell performance and useful cell life due to flooding of the active layer of the electrode.
In gas-generating cells, it is customary for the gas to be generated on the front face (electrolyte-side) of the electrode. The gas is thus generated as bubbles in the electrolyte, which can lead to removal of electrolyte from the cell and increased ohmic losses. Generation of gas in a gas diffusion electrode is more desirable, because the gas can exit the cell directly through the back of the electrode. Operation in this mode would require a certain amount of pressure tolerance. Even higher pressure tolerance would be required if the gas is generated in a pressurized state.
If the differential pressure between the gas and liquid sides of an electrochemical cell using a porous gas diffusion electrode is to be maintained at a low level, impractical pressure management problems result, especially in view of the fact that pressure levels vary from point to point on each side of the electrode.
The problems described are not readily amenable to correction by the use of a gas barrier material between the gas and electrolyte sides of the electrode, since such barriers tend to block the flow of electrolytic ions through the electrode and also strongly contribute to voltage losses or do not allow operation at a sufficiently high current density for the desired application. It is desirable to maintain the potential across the electrode at as positive a level as possible while maintaining as high a current density as possible. For example, it may be desired to operate a cell at a current density of up to as high as 500 mA/cm.sup.2, typically at 100 mA/cm.sup.2, while minimizing the voltage loss across the electrode. A voltage loss of less than 0.05 volts is preferred, with voltage losses of up to 0.25 volts being generally acceptable.
One approach to solving these problems is disclosed in Juda and Ilan U.S. Pat. No. 4,614,575 (Sep. 30, 1986), which involves the use of nonionic polymeric hydrogel as a layer applied by painting onto the electrolyte side of the gas diffusion electrode. The maximum pressure tolerance disclosed by the Juda, et al., patent is less than or equal to 40 inches of water (1.44 psi or 10.0 kPa), which is significantly less than that possible with the present invention.